Print heads employed in inkjet printers and the like usually contain a plurality of nozzles arranged in (an) array(s). The nozzles usually are placed substantially equidistant. The distance between two contiguous nozzles defines the nozzle pitch. In operation, the nozzles are controlled to the image-wise discharge of fluid droplets of a marking substance on an image-receiving member. When the printer is of the scanning type, the print heads are supported on a carriage which is moveable in reciprocation across the image-receiving member, i.e. the main scanning direction. In such printers, the print heads are typically aligned in the sub scanning direction perpendicular to the main scanning direction. In a traverse of the carriage across the image-receiving member a matrix of image dots of a marking substance, corresponding to a part of an original image is formed on the image-receiving member by image-wise activating selected nozzles of the print heads. The printed matrix is generally referred to as a print swath, while the dimension of this matrix in the sub scanning direction is referred to as the swath width. Usually, although not required, the printing swath is constant within a selected printing mode. When a part of the image is completed, the image-receiving member is displaced relative to the carriage carrying the print heads in the sub-scanning direction, enabling printing of a subsequent part of the image. When this displacement step is chosen equal to a swath width, an image can be printed in multiple non-overlapping swaths. An advantage of such approach is the high productivity as only a single printing stage is employed. However, the image quality may be improved by employing printing devices enabling the use of multiple printing stages. In the prior art two main categories of such printing devices can be distinguished, i.e. so-called “interlace systems” and “multi-pass systems”.
In an interlace system, the print head contains N nozzles, which are arranged in (a) linear array(s) such that the nozzle pitch is an integer multiple of the printing pitch. Multiple printing stages, or so-called interlacing printing steps, are required to generate a complete image. According to this disclosure, the print head and the image-receiving member are controlled such that in M printing steps, M being defined here as the nozzle pitch divided by the printing pitch, a complete image part is formed on the image-receiving member. After each printing step, the image-receiving member is displaced over a distance of M times the printing pitch. Such a system is of particular interest because it allows one to achieve a higher print resolution with a limited nozzle resolution.
In a “multi-pass system”, the print head is controlled such that only the nozzles corresponding to selected pixels of the image to be reproduced are image-wise activated. As a result an incomplete matrix of image dots is formed in a single printing stage or pass, i.e. one traverse of the print heads across the image-receiving member. Multiple passes are required to complete the matrix of image dots. In-between two passes the image-receiving member may be displaced in the sub scanning direction.
Both “interlace systems” and “multi-pass systems” as well as combinations thereof share the advantage of an improved image quality but also the inherent disadvantage of a lower productivity. In practice the majority of print jobs is executed in a multiple printing stage mode. Displacements between the image-receiving member and the carriage are executed in small increments, the increment usually being much smaller than a print swath width.
Often after being deposited on the image-receiving member, the image dots of a marking substance are subjected to irradiation by a radiation source which may be positioned laterally adjacent the carriage on the carriage itself or on a separate mount moveable in co-operation with the carriage. This may be done for several purposes including to prevent or control bleeding, to improve adhesion, in the case of a solvent based marking substance to remove the solvents, in the case of a radiation curable marking substance to set or cure the marking substance, etc. The radiation source(s) is (are) are mounted in such a way that all the marking substance deposited on the image-receiving member is exposed to radiation. For instance, in the case where the marking substance is an UV curable ink and the radiation source is a mercury vapor lamp, there is a minimum dose of energy that is required to cure the ink. As discussed above, in a multiple printing stage mode, the swath of ink jetted on the image-receiving member in one traverse of the carriage is typically much wider than the incremental displacement of the carriage relative to the image-receiving member. Hence, ink discharged from nozzles positioned on one side of the carriage in the sub scanning direction will be exposed to multiple doses of radiation while ink discharged from nozzles positioned on the opposite side of the carriage may only be exposed to a single dose of radiation originating from a single traverse of the lamp. As a consequence it may well be that the overall power output level of the lamp must be increased in order to ensure that all ink deposited, including the ink exposed by only a single traverse of the lamp, receives the minimum radiation dose required to cure the ink. Besides the fact that such ineffective use of additional power is environmentally unfriendly and costly, there may be some additional disadvantages associated with the use of an increased output power level. For instance, the increase in power level also results in an increase of heat which is particularly undesired when curing ink deposited on thermal sensitive image-receiving members. Moreover, part of the ink deposited is exposed to multiple traverses of the UV lamp, which output level is increased, and hence overcuring may occur as some inks are sensitive thereto.